Goldfish ... I think after years and years of aquariums in all houses where I was (even in the barracks and in the office) I have NEVER held a goldfish!
So my knowledge is limited, also because my father aged 6 to 12 years held in our house a large aquarium, and then adapted to contain tropical species, both freshwater and saltwater, marine.

Many people, perhaps from little ones, they kept a goldfish in the house, perhaps the classic reddish orange oranda, but never purple (Carassius auratus, family Cyprinidae) ... remember that even before Christ in China bred these animals for ornamental!
Now go out of fashion some "variations" on the subject as the "lion head" or "bubble eye", but in my humble opinion are "very obbbbbrobbbbiosi" and it would be better to stay away. Just watch them swim ... we can hardly yet have FISH, should spill from side to side without hesitation, should be their natural environment. Instead move as if they were finished in a tub of mud ...
While there are varieties instead "dappled" or fins longer, sometimes intriguing, funny and alternatives.

It 'a fish' rustic ', robust, omnivorous, his favorite foods are boiled zucchini, peas, blanched, boiled spinach, the bloodworms, artemia, daphnia and clear all the processed foods of the best-known companies. A' VARIOUS power is important for any fish, including goldfish.
It can safely be raised in open ponds or fountains, is a "cold-water fish", it needs very high temperatures, so we can avoid using a heater, negating the initial fee and then the consumer (which is not just trust me ...).
Fish are sensitive to temperature changes ... always much less than tropical fish, however, must be avoided 100% water changes with water taken directly from the tap ... which unfortunately happens often. When it says "water change", does not mean NEVER change TOTAL water of the tank, but ALWAYS a PARTIAL return of 10% minimum and up to 50%. So if the aquarium contains 20 liters, we should change MINIMUM 2 liters, preferably 5, and even better with the water conditioner added to the water or at least water left to stand for 24 hours.
Important to clean the bottom, sifonando when the water changes to make them less frequent and would also better equip the tank filtration, which is outside with the algae can also be avoided.
In small aquariums is very difficult to breed them with tropical plants, because of their size and strength of the goldfish, then break away and / or eat the leaves of poor seedlings. You could always try with plants very "tough" and in aquariums of at least 40-50 liters, for example Microsorium, Anubias, and perhaps some large Echinodorus.
The reproduction of the goldfish is not difficult, it is a fish of medium size that can give great satisfaction, while avoiding to leave him trapped in a fishbowl without any care, without friends, without furniture or plants without ... nothing!